Chinese scientists can monitor the polymerase chain reaction, a process used by molecular biologists and forensic scientists across the world, using a new approach to DNA detection. 

DNA sensors have many applications, including in disease diagnosis and environmental monitoring. The aim of the team led by Kui Jiao, at the Qingdao University of Science and Technology, was to develop a device for detecting genetically modified organisms. Their device combines an electrochemical sensor with the polymerase chain reaction (PCR), a method which amplifies target DNA sequences by repeated copying.

While PCR is a very sensitive technique, methods of analysing the result are not perfect, often requiring toxic chemicals or extra steps, which increase the sample contamination risk and the time and cost of analysis. Jiao's new system detects successful amplification simply by dipping an electrode into the post-PCR sample.

The nanotube-coated electrode records a reduced current in a successful polymerase chain reaction

Using an electrode coated with carbon nanotubes, the system determines the consumption of dGTP - one of DNA's building blocks - by the PCR. This is possible because the electrode can oxidise free dGTP, but is less able to oxidise dGTP when it is embedded in the DNA double helix. This results in a reduction in the system current proportional to the extent of the amplification reaction.

Jiao says the team had previously looked to the electrochemical properties of the finished DNA for their detector. 'But the need for a sensor with super-sensitivity, made us consider the other side of the coin,' explains Jiao, monitoring the loss of starting material rather than the build up of product.

The DNA detection arises because a PCR is started by adding DNA fragments complementary to parts of a target sequence. Only if the target sequence is present will the added fragments initiate the copying process, and amplification and the current decrease be observed. Using this method, the team has successfully identified transgenic maize samples, obtaining results agreeing with a traditional PCR analysis method.

Edmond Magner, who researches bioelectrochemistry at the University of Limerick, Ireland, sees the technique's potential. He warns there are a number of hurdles which will need to be overcome, such as producing carbon nanotubes reliably and cheaply. But once these challenges are met 'it could provide a relatively fast and possibly cost effective means of monitoring PCRs if it could be applied on a wider scale,' says Magner. 

Frances Galvin